1. Field
The present invention relates to a perpendicular magnetic recording medium for use in, e.g., a hard disk drive using the magnetic recording technique, and a perpendicular magnetic recording/reproducing apparatus.
2. Description of the Related Art
Recent hard disk drives have become smaller and smaller, and a product having a disk diameter of 1 inch. or less has been proposed. In addition, to realize an inexpensive, high-speed, large-capacity recording medium, the recording density is required to further increase. One of the most effective methods of increasing the areal recording density of a magnetic recording medium is to reduce noise by reducing the size of the magnetic crystal grains in the magnetic recording layer. Conventionally, in a method of stacking thin films by using sputtering, size is reduced by changing the thin film materials or optimizing the thin film formation conditions. However, in the method of simply stacking thin films, the grain size is often naturally determined and cannot be unconditionally controlled. For example, the average grain diameter of an underlayer below the magnetic recording layer is about 8 nm. Also, when a granular recording layer is used as the magnetic recording layer, the average grain diameter can be reduced to about 6 nm, but further reduction has been found to be very difficult.
Although it is being attempted to further reduce the crystal grain size of the magnetic crystal grains by using a granular structure in which the grains are segregated in an amorphous matrix, the crystal grains did not easily grow into columns, the crystal orientation readily deteriorated, and the crystal grain density was low by using this method. It is preferable that the degree of crystal orientation of the magnetic crystal grains is as high as possible, and the orientation variance is as low as possible. In the granular structure described above, however, the crystal orientation generally worsens and often becomes random, and this greatly increases the medium noise. Also, the crystal grains sometimes grow into spheres instead of columns. Even when the crystal grains grow satisfactorily into columns and relatively uniform nuclei form on the upper surface of the underlayer, some crystal grains often become enlarged, inhibit growth, or subsequently combine with other grains. As described above, it is generally difficult to grow magnetic recording crystal grains while a predetermined grain size is material in the direction of thickness of the magnetic recording layer.
Also, the magnetic recording layer is becoming thinner and thinner, and the signal output lower and lower. Therefore, if the crystal grain density greatly decreases, it is necessary to greatly increase the saturation magnetization of the magnetic crystal grains in order to maintain the output. To increase the saturation magnetization, a large change in composition is generally required. However, the segregated structure and crystal orientation are often disrupted, increases the medium noise, so this method is also not easy to put into practice.
In addition, as described in, e.g., Jpn. Pat. Appln. KOKAI Publication Nos. 2001-52330 and 2003-59037, it has been proposed to form small island-like nuclei on the underlayer or substrate, and epitaxially grow magnetic crystal grains or grow the grains into columns from these nuclei, thereby reducing the size of the grains. The size of the nuclei can be reduced in principle by reducing the formation amount of the nuclei. However, the individual crystal grains, having grown from the nuclei, enlarge in the in-plane direction until the gaps between adjacent nuclei are filled, unless the granular structure as described above is used. It seems possible, as a model, to imagine the high density of the nuclei. In practice, however, it is physically very difficult to realize a state in which fine nuclei spread such that they do not form a continuous film but separate from each other, in case of depositing, on the nuclei, magnetic crystal grains in an amount with which a continuous film can be formed. Generally, a continuous film is formed, or large islands are formed at low density.